Processes currently used in practice for phosphating zinc surfaces, for example electrolytically zinc-plated steel strip, still have certain limitations which would be desirable to eliminate. Thus, to guarantee adequate protection against corrosion, it is regarded as necessary for the phosphate layers to have weights per unit area of from about 2 to about 3 g/m.sup.2. These comparatively high weights per unit area frequently result in unsatisfactory or poor adhesion of subsequently applied layers, particularly when a phosphated and siccative coated material is subjected to forming processes. In the phosphating processes used in practice, the phosphating treatment lasts more than 5 seconds. With the processes used heretofore, it would be very difficult or even impossible to shorten the phosphating time, for example by increasing the speed of travel of the strips from 60 to 120 m/minute. Potential faults would include, for example, breaks in the phosphate layer applied and, hence, poorer protection against corrosion, and unsatisfactory forming and lacquer adhesion properties. Treatment times in the phosphating stage of less than 5 seconds cannot be obtained with the known processes used in practice.
GB Pat. No. 1,257,947, which relates to a process for applying thin, corrosion-resistant and firmly adhering zinc phosphate coatings to metal surfaces, proposes treating these metal surfaces, which have been galvanized, with an acidic solution to which has been added at least one carbohydrate consisting of starch, a starch derivative or a polysaccharide produced by the acidic decomposition of starch or a starch derivative. By spraying on phosphating solutions of this type, uniform zinc phosphate coatings having a low weight per unit area of, for example, from 1.2 to 1.8 g/m.sup.2 are said to be obtained over a period of from 3 to 10 seconds. The use of starch, starch derivatives or polysaccharides in the phosphating bath gives rise to considerable practical difficulties. These organic constituents are decomposed by the temperature and pH-value of the bath as the bath increases in age. The initially light phosphate coatings accordingly become distinctly heavier. The decomposition products release a strong odor. Extremely heavy sludge formation occurs which is particularly undesirable.
For forming thin, coherent phosphate coatings having a weight per unit area of less than 1.0 g/m.sup.2, U.S. Pat. No. 3,810,792 proposes treating the metal surfaces with solutions containing nickel ions as layer-forming cations. Layer-forming cations of another divalent metal, particularly zinc ions, can also be present in addition to the nickel ions. In the latter case, however, the molar ratio of nickel ions to the other divalent metal cations is distinctly greater than 1 whereas it is known that Ni:Zn ratios should be in the range of from 1:0.001 to 1:0.7. Using the solutions of this patent, mostly nickel phosphate layers are deposited. Accordingly, the zinc phosphate layers required in practice are not formed. In addition, the thin nickel phosphate layers obtained according to this patent are subject to serious limitations. Thus, they always have to be subsequently overcoated with other coating compositions to obtain adequate protection of the metal substrate.
The demand for improved protection against corrosion has today resulted in an increase in the use of electrolytically zinc-plated steel for many industrial products. At the same time, efforts are constantly being made in practice to operate existing installations at higher speeds for economic reasons. In the case of the processes currently used for phosphating electrolytically zinc-plated steel, these shortened treatment times have resulted in a distinct deterioration in the phosphate layer produced.